SO many factors contribute to line life one can't put a definate number on it.

To prolong any lines life you must take care of them. This not only means cleaning them after every trip but also making sure you don't step on your line. If you can help it, don't let your line rest on the ground during casting. Try not to drag your sinking lines on the bottom if you can help it. And most importantly, inspect all your guides on your rod and your line guide (if you have one) on your reel. Everything that touches your line should be smooth and free from dirt and debris. After you clean your line and reel and put them away, clean and inspect your rod. It takes and extra 30 seconds to wipe it down and look at it as it slides into the rod bag. With just a little extra effort your lines will last longer and perform better and that makes spending money on the next one much easier.